Magnetoresistive devices, such as giant magnetoresistive (GMR) devices, are used in magnetic data storage systems to detect magnetically encoded information stored on a magnetic data storage medium such as a magnetic disc. A time dependent magnetic field from a magnetic medium directly modulates the resistivity of the GMR device. A change in resistance of the GMR device can be detected by passing a current through the GMR device and measuring the voltage across the GMR device. The resulting signal can be used to recover the encoded information from the magnetic medium.
A typical GMR device configuration is the GMR spin valve, in which the GMR device is made of a non-magnetic spacer layer positioned between a ferromagnetic pinned layer and a ferromagnetic free layer. The magnetization of the pinned layer is fixed in a predetermined direction, typically normal to the air bearing surface (ABS) of the GMR device, while the magnetization of the free layer rotates freely in response to an external magnetic field. The resistance of the GMR device varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the pinned layer. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect, i.e. greater sensitivity and higher total change in resistance, than is possible with anisotropic magnetoresistive (AMR) devices, which generally consist of a single ferromagnetic layer.
GMR spin valves are configured to operate either in a current-in-plane (CIP) mode or a current-perpendicular-to-plane (CPP) mode. In CIP mode, the sense current is passed through the device in a direction parallel to the layers of the device. In the CPP mode, a sense current is passed through the device in a direction perpendicular to the layers of the device.
A tunneling magnetoresistive (TMR) device is similar in structure to a GMR spin valve configured in CPP mode, but the physics of the device are different. For a TMR device, rather than using a spacer layer, a barrier layer is positioned between the free layer and the pinned layer. Electrons must tunnel through the barrier layer. A sense current flowing perpendicular to the plane of the layers of the TMR device experiences a resistance that is proportional to the cosine of an angle formed between the magnetization direction of the free layer and the magnetization of the pinned layer.
As the need continues for higher areal density for recording heads, higher density memory elements and smaller magnetic sensors, the size of magnetoresistive devices continues to decrease. However, as the size of magnetoresistive devices decreases, the variation in magnetization direction of the pinned layer increases. For example, a 350 nanometer (nm) sensor has a standard deviation of its pinning angle of about 2.5 degrees. However, for a sensor of about 50 nm in size, the standard deviation of its pinning angle increases to about 13 degrees.
One solution to this problem is to increase the size of the pinned layer in relation to the free layer. For example, U.S. Pat. No. 6,762,915 discloses a read sensor for a magnetic read head with a free layer, a pinned layer, and a pinning layer, wherein the pinned and pinning layers each have a greater lateral size than the free layer and a greater size perpendicular to an air-bearing surface than the free layer.
However, there remains a need for enhanced stability of the magnetization direction of the pinned layer in magnetoresisitive stacks. Therefore, it is desirable to develop magnetoresistive stacks with increased stability of the magnetic orientation of the pinned layer.